Why can’t I join or complete jobs

There could be a few reason for this.

First, make sure that the race is configured to allow custom cars. The lobby's host can disable custom cars if they want. Though typically, I don't run into many lobbies with customs disabled. I am level 70+, so I end up in lobbies where most people have custom cars of every type. It may be more common for customs to be disabled in lower leveled lobbies.

Second, it could be a timing issue. After you buy a car it takes a few minutes for it to be available in your garage. If it hasn't been delivered yet, you won't be able to race with it. I've also seen it take a while for purchases to propagate though their cloud servers. So it could be a case that the session that the race lobby lives in doesn't know about your new car yet. I wouldn't be concerned unless it's not available by the next day.

You have to join the crew through the Rockstar Social Club. If you haven't done so, create an account and link it to your PS3 gamer profile. This should add GTA V as a game in your social club profile.

You'll get a GTA V career page to navigate to once you've done that. There it shows all three of the single player characters and your online character. If you click on your online character just below the thumbnail image of that character you'll see their current cash and a big "Join a Crew" button.